We have identified two genes from Arabidopsis that show high similarity with CBF1, a gene encoding an AP2 domain-containing transcriptional activator that binds to the low-temperatureresponsive element CCGAC and induces the expression of some cold-regulated genes, increasing plant freezing tolerance. These two genes, which we have named CBF2 and CBF3, also encode proteins containing AP2 DNA-binding motifs. Furthermore, like CBF1, CBF2 and CBF3 proteins also include putative nuclear-localization signals and potential acidic activation domains. The CBF2 and CBF3 genes are linked to CBF1, constituting a cluster on the bottom arm of chromosome IV. The high level of similarity among the three CBF genes, their tandem organization, and the fact that they have the same transcriptional orientation all suggest a common origin. CBF1, CBF2, and CBF3 show identical expression patterns, being induced very rapidly by low-temperature treatment. However, in contrast to most of the cold-induced plant genes characterized, they are not responsive to abscisic acid or dehydration. Taken together, all of these data suggest that CBF2 and CBF3 may function as transcriptional activators, controlling the level of low-temperature gene expression and promoting freezing tolerance through an abscisic acid-independent pathway.

The higher plant Arabidopsis thaliana (Arabidopsis) is an important model for identifying plant genes and determining their function. To assist biological investigations and to define chromosome structure, a coordinated effort to sequence the Arabidopsis genome was initiated in late 1996. Here we report one of the first milestones of this project, the sequence of chromosome 4. Analysis of 17.38 megabases of unique sequence, representing about 17% of the genome, reveals 3,744 protein coding genes, 81 transfer RNAs and numerous repeat elements. Heterochromatic regions surrounding the putative centromere, which has not yet been completely sequenced, are characterized by an increased frequency of a variety of repeats, new repeats, reduced recombination, lowered gene density and lowered gene expression. Roughly 60% of the predicted protein-coding genes have been functionally characterized on the basis of their homology to known genes. Many genes encode predicted proteins that are homologous to human and Caenorhabditis elegans proteins.

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Phylogenetic studies of Trypanosoma cruzi have identified the existence of two groups: T. cruzi I and T. cruzi II. There are aspects that still remain unknown about the genetic variability within the T. cruzi I group. Given its epidemiological importance, it is necessary to have a better understanding of T. cruzi transmission cycles. Our purpose was to corroborate the existence of haplotypes within the T. cruzi I group and to describe the genetic variability and phylogenetic relationships, based on single nucleotide polymorphisms (SNPs) found in the miniexon gene intergenic region, for the isolates from different hosts and epidemiological transmission cycles in Colombian regions. 31 T. cruzi isolates were molecularly characterized. Phylogenetic relationships within T. cruzi I isolates showed four haplotype groups (Ia–Id), associated with their transmission cycle. In previous studies, we reported that haplotype Ia is mainly associated with the domestic cycle and domiciliated Rhodnius prolixus. Haplotype Ib is associated with the domestic cycle and peridomestic cycle, haplotype Ic is closely related with the peridomestic cycle, and haplotype Id is strongly associated with the sylvatic cycle. The phylogenetic methodologies applied in this study are tools that bolster the associations among isolates and thus shed light on Chagas disease epidemiology.

Type 2A serine/threonine protein phosphatases (PP2A) are important components in the reversible protein phosphorylation events in plants and other organisms. PP2A proteins are oligomeric complexes constituted by a catalytic subunit and several regulatory subunits that modulate the activity of these phosphatases. The analysis of the complete genome of Arabidopsis allowed us to characterize four novel genes, AtBЈ⑀, AtBЈ, AtBЈ, and AtBЈ, belonging to the PP2A BЈ regulatory subunit family. Because four genes of this type had been described previously, this family is composed of eight members. Reverse transcriptase-polymerase chain reaction experiments showed that AtBЈ⑀ mRNAs are present in all Arabidopsis tissues analyzed, and their levels do not respond significantly to heat stress. Expressed sequence tags corresponding to AtBЈ, AtBЈ, and AtBЈ have been identified, indicating that the new genes are actively transcribed. The genomic organization of this family of PP2A regulatory subunits is reported, as well as its chromosomal location. An extensive survey of the family has been carried out in plants, characterizing BЈ subunits in a number of different species, and performing a phylogenetic study that included several BЈ regulatory proteins from animals. Our results indicate that the animal and plant proteins have evolved independently, that there is a relationship between the number of BЈ isoforms and the complexity of the organism, and that there are at least three main subfamilies of regulatory subunits in plants, which we have named ␣, , and .Reversible protein phosphorylation is widely accepted as a major mechanism for the control of biological processes in eukaryotic cells. In plants, reversible protein phosphorylation is involved in processes such as hormonal, pathogenic, or environmental stress responses (Mumby and Walter, 1993;Smith and Walker, 1993; Garbers et al., 1996;Schö ntal, 1998; Janssens and Goris, 2001). In this context, Ser/Thr protein phosphatases (PPs) are important regulatory components of many signal transduction pathways (Ingebritsen and Cohen, 1983a;Schö ntal, 1998). Several Ser/Thr phosphatases, grouped into different categories, have been identified in a variety of plant species. Specifically, homologs of the 1, 2A, and 2C types of animal PPs have been described in plants (Rodríguez, 1998; Lin et al., 1999; Meek et al., 1999). All these types of PPs are distinguished by their different sensitivity to inhibitors and their divalent cation requirements, and are structurally different (for review, see Mumby and Walter, 1993).Type 2A phosphatases (PP2A) are oligomeric enzymes with no obvious requirements for ions or cofactors, and are implicated in a variety of cellular processes (Mumby and Walter, 1993; Janssens and Goris, 2001). In general, the native forms of PP2A proteins exist as oligomeric complexes, constituted by a catalytic subunit (PP2Ac), and one or more regulatory subunits named A and B. Thus, PP2A proteins can be heterodimers, consisting of a PP2Ac catalytic subunit and a type A...